NH4 for K substitution in dioctahedral mica synthesized at 200 °C

Abstract Synthesis of intermediate K-NH 4 dioctahedral micas from kaolinite had been carried out using two chemical systems K 2 O-(NH 4 ) 2 O-Al 2 O 3 -SiO 2 -H 2 O (1) and K 2 O-(NH 4 ) 2 O-Al 2 O 3 -SiO 2 -HCl-H 2 O (2). We have used variable NH 4 :K ratios, with the aim of testing the existence of a possible miscibility gap between the NH 4 - and the K-end members at 200 °C. Reactions in system (1) mainly produced zeolites. In system (2), a complete series of mica (except for the molar NH 4 :K ratio = 0.0:1.0), were obtained. The results based on the X-ray diffraction patterns and Fourier Transform Infrared spectra indicated, in contrast with previous works, that a linear correlation between the average NH 4 content in mica and either the basal spacing or the area of the main NH 4 infrared band does not exist in the series studied. Thus, the direct estimation of the NH 4 in mica from these methods seems questionable. Indeed, the current NH 4 content in mica appears to be controlled by several factors including, in addition to the initial NH 4 content in the system, the presence of poorly crystalline phases, which is favoured in NH 4 -poor systems, and the equilibrium between the solid products and the solutions. Data for discrete mica particles obtained from the transmission-analytical microscopic study suggested that a miscibility gap exists in the range ~((NH 4 ) 20  K 80 –((NH) 4 ) 60 K 40 at 200 °C. This is analog to the gap observed in solid solutions of paragonite and muscovite.